GaN, AlN, InN, which are nitride semiconductors, or materials made of mixed crystals thereof, have a high saturated electron speed and a wide band gap, and studies are made to apply these materials to high breakdown voltage/high output electronic devices. As high breakdown voltage/high output electronic devices, technologies are developed in relation to Field effect transistors (FET), more particularly, High Electron Mobility Transistors (HEMT).
As a HEMT using a nitride semiconductor, there is an example in which the electron transit layer is formed with GaN and the electron supply layer is formed with AlGaN. In a HEMT having this structure, high density 2DEG (two-dimensional electron gas) is generated according to a distortion caused by the lattice constant difference between GaN and AlGaN, i.e., a so-called piezo polarization. Therefore, a high-efficiency, high-output semiconductor device is attained.
In a HEMT having a structure in which the electron transit layer is made of GaN and the electron supply layer is made of AlGaN, high density 2DEG is generated, and therefore it is difficult to make the HEMT become normally-off. In order to solve this problem, there is disclosed a method of forming a recess by removing a part of the electron supply layer in the area where the gate electrode is to be formed, and making the 2DEG disappear immediately below the gate electrode (see, for example, patent document 1). Furthermore, there is disclosed a method of forming a p-GaN layer between the gate electrode and the electron supply layer, and reducing the generation of 2DEG immediately below the gate electrode, so that the HEMT becomes normally-off (see, for example, patent document 2.
Patent document 1: Japanese Laid-Open Patent Publication No. 2009-76845
Patent document 2: Japanese Laid-Open Patent Publication No. 2007-19309
However, with the method of forming a recess, the damage caused by etching when forming a recess is applied near the electron transit layer, and therefore the on resistance increases and the leak current increases, which leads to deterioration in properties of the HEMT.
Furthermore, when forming a p-GaN layer between the electron supply layer and the gate electrode, generally, the p-GaN layer is formed by forming a p-GaN film on the entire surface of the electron supply layer, and then removing, by dry etching, the p-GaN film from areas excluding the area where a gate electrode is to be formed. However, GaN is a material that is significantly difficult for performing dry etching. Furthermore, it is not possible to perform etching by a high selection ratio between AlGaN and GaN, and therefore, it is difficult to remove only the p-GaN layer. Thus, in the area where the p-GaN layer is to be removed, there are cases where a part of the p-GaN layer remains, or cases where a part of the electron supply layer is removed from the area from which the p-GaN layer is to be removed. In these cases, the on resistance becomes high, and therefore properties of the HEMT are deteriorated. Furthermore, when variations arise in the thickness of the electron supply layer due to variations in etching performed by dry etching, variations arise in the properties of the HEMT, which leads to a decrease in yield ratio.
In the HEMT, when even a part of the electron supply layer is removed, the properties significantly deteriorate. Therefore, in order to prevent the electron supply layer from being damaged, the etching is performed in a state where a part of the p-GaN layer is remaining. However, if a part of the p-GaN layer remains in the area from which the p-GaN layer is to be removed, problems arise in addition to the above. That is, holes are generated in parts where the intervals between bands are discontinuous between the electron supply layer and the p-GaN layer, and a leak current is generated between the source and the drain.